1. Field of the Invention
The present invention relates to a protection circuit for protection from electro static discharge (ESD) and to a semiconductor device.
2. Description of the Related Art
A known example of semiconductor devices fabricated using semiconductor technology is display devices. Examples of such display devices include liquid crystal display devices and organic electroluminescent (EL) display devices.
A liquid crystal display device, in which a liquid crystal is sandwiched between a pair of electrodes in each pixel, displays an image by controlling the amount of light from the backlight on its rear side passing through the pixel, using light polarization produced by the liquid crystal. Examples of liquid crystal display devices include passive-matrix liquid crystal display devices and active-matrix liquid crystal display devices using thin film transistors (TFTs).
Organic EL elements have been actively researched and developed. A general organic EL element is composed of a pair of electrodes and a layer containing a light-emitting organic compound sandwiched between the electrodes. When voltage is applied to the element, the light-emitting organic compound emits light.
Like a liquid crystal display device, a display device using organic EL elements employs passive-matrix addressing, active-matrix addressing, or the like. An organic EL element, which is a self-light-emitting element that can be formed in a film shape, does not require a backlight unlike a liquid crystal display device and the like, resulting in a thin lightweight high-contrast display device that consumes less power. Patent Document 1, for example, discloses an example of a display device using organic EL elements.
However, when external pulse potential due to ESD is applied to such a display device through an external input terminal, display quality decreases due to noise and internal circuitry malfunctions. Moreover, when very high potential due to ESD is applied to the display device, a functional element in the internal circuitry may be damaged. Such potential due to ESD may be applied to not only the external input terminal, but also a direct input line or a wiring connected to pixels and the like.
As a device for solving the above problems, a protection circuit that protects internal circuitry from pulse potential due to ESD is known. Patent Document 2 discloses a technique to configure a protection circuit using thin film transistor technology.